State of charge (to be abbreviated as SOC) detection apparatuses are available that detect a SOC (also referred to as charging rate or residual capacity) of a battery used in a vehicle. For example, many SOC detection apparatuses that detect battery SOC of an electric vehicle (EV) detect SOC by integrating battery current. Although regenerative braking can be expected to provide temporary charging in EVs, the battery is discharged for the most part during vehicle operation. The SOC is restored by charging the battery when the vehicle is not operating. Thus, there are many cases in EVs in which the SOC detection apparatus detects the SOC by integrating discharge current from the time the battery is fully charged.
Integration of battery current is also frequently used to detect battery SOC in hybrid vehicles (HV) equipped with an engine and generator. However, in HVs, charging and discharging are controlled so as to maintain battery SOC at about 50% from the viewpoints of battery service life and the acceptable amount of regenerated electrical power. Thus, there are few opportunities for the battery to become fully charged (to nearly 100%), or in other words, there are few opportunities for initializing the SOC. If detection of the battery SOC is only carried out by integrating charge and discharge current over a long period of time, current detection error is accumulated and error in the detected SOC ends up increasing.
The SOC detection apparatus described in Japanese Patent Application Publication No. 2008-241246 (JP-A-2008-241246) estimates SOC using an internal reaction model. In this SOC detection apparatus, SOC is estimated from the relationship between open circuit voltage (OCV) and SOC by utilizing the fact that the OCV of the secondary battery has a slope.
FIG. 26 is a graph that shows an SOC-OCV curve of a battery that demonstrates characteristics having a slope. With reference to FIG. 26, OCV (V) is plotted on the vertical axis and SOC (%) is plotted on the horizontal axis. For example, a lithium ion battery that uses a nickel-based or cobalt-based active material as an electrode demonstrates characteristics like those shown here. According to the characteristics shown in FIG. 26, the SOC-OCV curve has a slope, and OCV and SOC demonstrate a 1:1 relationship. Thus, SOC can be estimated by determining OCV in consideration of such factors as battery terminal voltage, battery temperature or battery current. As a result, the estimation accuracy of SOC can be improved even in cases in which there are few opportunities for the battery to become fully charged.
In recent years, olivine batteries (olivine lithium ion batteries) have attracted attention for their high level of safety and low price. Olivine batteries are a type of lithium ion battery, and this combined with the anode to create a region where the battery voltage is flat (plateau region). A material such as carbon is used for the anode. Since olivine batteries offer lower costs and higher safety than cobalt-based electrode materials, they have attracted attention in recent years as large-scale batteries in the manner of electricity storage devices used in EVs and PHVs. This type of olivine battery is disclosed in, for example, Japanese Patent Application Publication No. 2002-117833 (JP-A-2002-117833) and Japanese Patent Application Publication No. 2005-302300 (JP-A-2005-302300).
However, olivine batteries have a long plateau region (region in which OCV remains roughly constant even though SOC changes), thus making it extremely difficult to estimate SOC from values of OCV.
FIG. 27 is a graph that shows an SOC-OCV curve of an olivine battery. With reference to FIG. 27, OCV (V) is plotted on the vertical axis and SOC (%) is plotted on the horizontal axis. As shown in FIG. 27, since a lithium ion battery that uses an olivine active material as an electrode has a long plateau region, it is extremely difficult to estimate SOC from values of OCV. For example, in an example of a battery that uses a carbon-based material for the anode, there are cases in which the plateau region covers a range of SOC values of 30% to 95%.